In a cooperative study with the Hospital de Clinicas in Montevideo, Uruguay, 12 patients with chronic myelogenous leukemia in blast crisis were treated with combination chemotherapy consisting of 5-Azacytidine (150 ml/sq.m IV x 5 days) and VP16-213 (74 ml/sq.m IV/day/x 5 days). Eight evaluable patients to date one partial response and six definite antileukemic effects have been seen. Five of eight patients have died with a median overall survival of 50 days (20-135 plus days). Side effects consisted of nausea, vomiting and muscular ache with severe mucositis occurring in one patient. Although only a relatively small number of patients have been treated, the initial response rate has been disappointing and in keeping with the notorious refractory nature of blast crisis of CML.